The present invention relates to semiconductor packages, and more particularly, to a semiconductor package having a heat sink in which a top surface of the heat sink is exposed to the outside of the package for enhancing the heat-dissipating efficiency.
A BGA (ball grid array) semiconductor package is advantageous for having sufficient I/O connections as required for a semiconductor chip having high density of electronic components and electrical circuits. Accordingly, heat is expected to be generated in a huge amount during operating such a densely equipped semiconductor chip. In other words, the heat dissipation is critical for maintaining the performance and lifetime of the semiconductor chip. However, as the conventional BGA semiconductor package typically has the semiconductor chip thereof encapsulated by an encapsulant, the heat usually can not be effectively dissipated to the atmosphere through the encapsulant, which is made of a molding resin having a small coefficient of thermal conductivity K being only about 0.8 w/mxc2x0K.
In addition, after forming the encapsulant for encapsulating the semiconductor chip, since the semiconductor chip has a coefficient of thermal expansion (CTE) about 3 ppm/xc2x0C. much smaller than that of the molding resin about 20 ppm/xc2x0C., the encapsulant has relatively greater extent in thermal expansion and cold shrinkage corresponding to significant temperature variation during a curing process for curing the encapsulant, a solder reflow process for soldering the semiconductor package on a printed circuit board, and a reliability test for the semiconductor package in a temperature cycle. Accordingly, certain thermal stress effect is generated from the encapsulant on the semiconductor chip, resulting in cracks in the semiconductor chip. Therefore, quality and production yield of the semiconductor package can not be assured.
In order to solve the foregoing problem of ineffectiveness in the heat dissipation, a BGA semiconductor package having a heat sink is proposed. Such a semiconductor package helps increase the heat-dissipating efficiency, however, since the heat generated from the semiconductor chip needs to be transmitted for a long path through the encapsulant with poor thermal conductivity to the atmosphere, the overall heat-dissipating efficiency of the semiconductor package is not satisfactory.
In accordance with the drawback depicted in the above BGA semiconductor package, U.S. Pat. No. 5,216,278 discloses a semiconductor package with a heat sink in which a top surface of the heat sink is exposed to the outside of the semiconductor package. As shown in FIG. 5, the semiconductor package 1 has the heat sink 10 thereof attached to a top surface of a semiconductor chip 12 through a thermally conductive adhesive layer 11, and an encapsulant 13 is formed for encapsulating the chip 12 in a manner that the top surface of the heat sink 10 is exposed to the outside of an encapsulant 13. This makes heat generated from the chip 12 dissipated to the atmosphere through a thermally conductive path constituted by the thermally conductive adhesive layer 11 and the heat sink 10 excluding the encapsulant 13, so that the heat-dissipating efficiency of the semiconductor package 1 can be greatly improved. However, as the heat sink 10 is directly attached to the top surface of the chip 12, the chip 12 is also subjected to a clamping effect generated by an encapsulating mold (not shown) on the heat sink 10 during a molding process, which makes the chip 12 cracked and quality of the semiconductor package 1 degraded. Furthermore, as the semiconductor chip has the CTE about 3 ppm/xc2x0C. much smaller than that of copper about 18 ppm/xc2x0C. used for making heat sink 10, the heat sink 10 induces a significantly thermal stress effect on the chip 12, and similarly, cracks occur in the chip 12 and production yield of the semiconductor packages 1 is degraded.
According to the defects depicted in the above semiconductor package, the present inventor proposes a semiconductor package having a heat sink in Taiwanese patent application No. 87116851. As shown in FIG. 6, the semiconductor package 2 has the heat sink 20 thereof similarly constructed as the heat sink in the foregoing semiconductor package, that is, a top surface 200 of the heat sink 20 is exposed to the atmosphere for enhancing the heat-dissipating efficiency of the heat sink 20. Moreover, a bottom surface 201 of the heat sink 20 is properly spaced apart from a semiconductor chip 22 for preventing the heat sink 20 from clamping the chip 22 during molding. As such, a molding resin used for forming an encapsulant 23 fills the space between the heat sink 20 and the chip 22, allowing heat generated by the chip 22 to be transmitted through the encapsulant 23 for dissipation, which definitely degrades the heat-dissipating efficiency of the heat sink 20 as previously described in the prior art. In addition, as the chip 22 is directly encapsulated by the encapsulant 23, a thermal stress effect from the encapsulant 23 on the chip 22 is induced, and thus the chip 22 may be damaged by cracking.
A primary objective of the present invention is to provide a semiconductor package in which a lid is attached to a semiconductor chip and appropriately spaced apart from a heat sink having a top surface thereof exposed to the outside an encapsulant, so as to prevent external moisture from condensing on the semiconductor chip and reduce a thermal stress effect on the semiconductor chip, as well as avoid cracks in the semiconductor chip in a temperature cycle. Moreover, a thermal conductive path is reduced in a portion passing through the encapsulant, allowing the heat-dissipating efficiency to be improved. In addition, with no contact between the heat sink and the semiconductor chip, quality of the semiconductor package is assured with no damage to the semiconductor chip.
According to the above and other objectives, a semiconductor package proposed in the present invention includes: a substrate having a first surface and a second surface; a semiconductor chip having a first surface and a second surface, while the second surface of the chip is attached to the first surface of the substrate; a plurality of first conductive members for electrically connecting the chip to the substrate; a lid attached to the first surface of the chip, and made of a material having a coefficient of thermal expansion similar to that of the chip; a heat sink mounted on the first surface of the substrate, and having a first surface and a second surface, while a gap is formed between the second surface of the heat sink and the lid; a plurality of second conductive members for electrically connecting the chip to an external device; and an encapsulant for encapsulating the chip, the lid, the first conductive members and the heat sink, while the first surface of the heat sink is exposed to the outside of the encapsulant.
In another embodiment of the invention, the semiconductor chip has the first surface thereof electrically connected to the substrate through solder bumps in a flip chip manner, and accordingly the lid is attached to the second surface of the chip.
The lid is made of a material having a coefficient of thermal expansion similar to that of the semiconductor chip, preferably made of a semiconductor material or a metallic material which can effectively transmit the heat generated by the semiconductor chip connected with the lid. More preferably, the lid is made from a defective wafer such that the lid has the same coefficient of thermal expansion as that of the semiconductor chip. Thus, a thermal stress effect on the first surface of the semiconductor chip in a temperature cycle can be minimized.
Furthermore, the gap between the lid and the heat sink is preferably from 0.03 mm to 0.45 mm, and more preferably from 0.05 mm to 0.30 mm. If the gap is too big, the rather thick encapsulant formed in the gap will detrimentally affect the heat-dissipating efficiency. If the gap is too small, the flow of a molding resin injected to the gap will slow down due to the increased resistance, which may result in the formation of voids between the lid and the heat sink. As a result, the voids tend to cause a popcorn effect in the semiconductor package in the temperature cycle, a reliability test and the actual operation, and thus quality of products is degrade. Moreover, the voids also lead to increase in the thermal resistance since the thermal conductivity of air is poorer than that of the encapsulant, so that the heat-dissipating efficiency will be reduced.
In addition, in order to further minimize the gap for reducing the overall thickness of the fabricated semiconductor package without the formation of the voids, on the first surface of the lid along a flow direction of the molding resin during molding there can be formed a plurality of grooves or flow channels for leading the flow of the molding resin, wherein the flow channels are built up between protrusions formed on the first surface of the lid. Similarly, the foregoing grooves or flow channels can also be formed on the second surface of the heat sink, or can be simultaneously formed on the second surface of the heat sink and the first surface of the lid.